"Scientists have been studying scar formation for over a century", says Dr Christian Göritz, who led the research. "A skin scar doesn't interfere with function, but in the brain and spinal cord a scar is a big problem: it can permanently interrupt nerve function."

Under the microscope, a scar in the spinal cord shows two cell types. There is a core of connective tissue cells called fibroblasts. Outside the core is a layer of cells, which are normally intimately involved with nerves, called astrocytes. Up till now, almost all research has focussed on the astrocytes, says Göritz.

The source of the fibroblast cells has been unknown, but Göritz's team has revealed that they arise from cells called pericytes that are normally wrapped round tiny blood vessels.

Pericytes are found throughout the body, hugging the outside of capillaries - the smallest blood vessels. They help maintain the blood-brain barrier and they have a key role in regulating capillary blood flow.

'Pericytes to the rescue'

Göritz's group was surprised to find that, when they damaged the spinal cords of mice, some of the perictyes took on a completely different role.

"[The pericytes] leave their normal position and move out into the damaged tissue," says Göritz. "They only do this in the lesioned tissue. In the normal tissue they still stick to the blood vessels".

"As soon as they reach the damaged tissue they become fibroblasts and they cluster together over time to seal the lesion from the core."

The cells also proliferate, rapidly producing more pericytes. Nine days after the injury, the number of pericytes in the injured area was 25 times normal.

It seems the pericytes have a role rather like Clark Kent in Superman, quietly getting on with their day job until a spinal injury happens, and then it's 'pericytes to the rescue'.

The researchers went on to show that the pericytes were essential for the wound to seal. If they genetically modified them so they were unable to reproduce, the wound would remain an open hole.

"Scars are both good and bad", says Göritz. They fill and seal the wound, but the scar prevents the spinal cord from working properly.

"We'd like to find ways to modify it [the scarring process] to keep the good things and change the bad ones," says Göritz

He says pericytes may be involved in scar formation in other organs and also in fibrosis, a disease where there is overproduction of connective tissue.

"Capillaries and pericytes are found all around the body," says Göritz. "It could be a general mechanism".